A PREDICTIVE MODEL FOR HYDROGEN IN STEEL IN NON-DEGASSED HEATS

A PREDICTIVE MODEL FOR HYDROGEN IN STEEL IN NON-DEGASSED HEATS 

ABSTRACT

Hydrogen may cause several problems during steel processing. Defects caused or enhanced by hydrogen range from different types of bubbles such as pin-holes to break out during continuous casting. Further down the line, segregation and embrittlement may lead to cracking such as flaking or blistering. These problems impact plant productivity and have cost impacts on equipment maintenance and the need for additional steel treatment. Some of the problems lead to scraping. Although vacuum degassing effectively controls the hydrogen content of steel, it introduces additional costs that are not justifiable for many products.

This work aims first to identify the main sources of hydrogen in liquid steel in the Ternium Brazil steelmaking plant and propose a model to decide the need for hydrogen measurement for the degassing process, focusing on steels for which vacuum degassing is not a specification requirement. It is essential for these steels to guarantee a controlled level of dissolved hydrogen to avoid problems, mostly at the casting. Once these sources are identified, a model is developed to predict the hydrogen content at the beginning of the secondary metallurgy treatment. Based on the model, it is proposed that hydrogen should be measured or not at this step to decide if vacuum degassing is required to assure safety in casting.

Keywords: hydrogen, steel, steel plant.

1. INTRODUCTION

The solubility of hydrogen varies with temperature and with phase transformations, as shown in Figure 1. In general, there is good agreement between the various published values of the solubility of hydrogen in steel. [1-3]

[pic 1]

Figure 1 - Solubility of hydrogen in iron (and low alloy steels) at 1atm as a function of temperature. Adapted from [1].

However, the hydrogen content in the atmosphere is only around 0,6ppm [4]. It has been well established that the main source of hydrogen in steel is the reduction of water, normally present as humidity [5-7], in accordance with Eq. 1. This will be discussed in more detail in the next section.

                                                   (1)[pic 2]

The high mobility of hydrogen atoms in steel causes very fast redistribution and segregation during phase change (e.g. [8]).  It has been established that the change in solubility during the  transformation is important to the formation of pin-holes and similar defects during solidification [9,10]. Currently, this is relatively well modelled [11]. The diffusion of hydrogen through, in the early stages of continuous casting seems to play an important in the occurrence of breakout in continuous casting of steels containing somewhere over 6-8ppm hydrogen [12,13]. Apparently, a larger volume of hydrogen diffuses through the initial thin layer of solid steel than is retained in pinholes or bubbles in general [14] influencing heat transfer, powder behaviour and eventually leading to breakouts.  This is a cause of concern in many steel mills [12,13,15].  Further down in the processing route, hydrogen may cause cracks, blisters [16] and flakes [17]. These phenomena are mostly associated with the combination of transformation stresses, alloying element segregation and hydrogen redistribution and are well understood [17-19], albeit they continue to be hard to control in many industrial situations. Hydrogen absorption during application and problems caused by it is also a very important problem [20], but this is not discussed here.[pic 3][pic 4]

Since the 1950´s the classical solution to control hydrogen content in steelmaking is vacuum degassing [18,21]. This solution, however, introduces significant additional costs. Thus, it should be limited to essential cases. The introduction of “Sievert´s Law-based” fast and reliable analysis of hydrogen (pioneered by HYDRIS® [22]) made possible the rapid analysis of hydrogen during steelmaking. This can be used as a decision tool concerning further processing of heats. However, this analysis also introduces a significant cost to the process.  Currently, Ternium systematically vacuum treats steels in which this is essential either because of client specification, chemical composition, final product thickness or application. For other steels, hydrogen is determined at the beginning of the secondary metallurgy step and, should it exceed a defined threshold, the heat is degassed regardless of other requirements.

Understanding the sources of hydrogen in the process as well as how hydrogen is absorbed or removed in processing steps in the melt shop can help reduce some of the costs in this process.  Two strategies have been followed in steel plants to prevent hydrogen problems while controlling processing cost: (a) Evaluating the sources of hydrogen in the steelmaking process (e.g. [6,7,10,23-25]). (b) creating models to estimate the hydrogen content at some step of the melt shop process (e.g. [13,26,27]). These strategies may be combined.  In the present case, we followed the combined approach: by identifying the most relevant sources of hydrogen in the steel when it reaches the secondary metallurgy, qualitative and semi-quantitative control measures can be proposed in raw materials and processing to prevent high hydrogen contents. The identification step is also relevant in helping defining which phenomena are relevant to be worth modeling.  We opted for kinetic models of some stages of the process, as opposed to using artificial intelligence, neural network and other efficient fitting tools. It was felt that by adhering to fundamentals, extrapolation to new conditions would be more reliable.  The main objective of the work was then twofold: indicate process steps/materials that are more relevant for hydrogen pickup at Ternium and propose a model that could reduce the need for analyzing the hydrogen of all heats when they reach secondary metallurgy. The combination of these two measures should reduce processing costs by reducing the number of heats with excessive hydrogen content and reducing the number of heats that must be analyzed to decide if they should be degassed or not.

1.  HYDROGEN SOURCES

As the absorption of hydrogen in steel occurs in accordance with Eq. 1, water, water vapour and humidity are fundamental concerns during steelmaking when hydrogen control is considered. In some cases, like oil contamination in scrap and coke additions, hydrogen may be present in a non-oxidized formed. And in ferro-alloys, besides humidity, dissolved hydrogen is also present as will be discussed.  Numerous sources of water and hydrogen must be considered in the melt shop. In this study, we consider sources in steps from converter charging until the start of ladle metallurgy, as indicated in the schematic meltshop flow of Figure 2.